The present invention relates to gyroscope systems such as those used for the control and navigation of aircraft and missiles.
A commonly used form of gyroscope system has a null-seeking servo with a type II control loop which is designed to be unconditionally stable. This arrangement allows the gyroscope to be operated in a "strap-down" mode, without needing gimbals to suspend it. The suspension of the gyroscope's rotor allows the rotor to move freely over a limited range of angles. The control loop then detects the angle of tilt of the rotor with respect to its null position and generates a torque to precess the rotor back into position. Since the tilt is generated by the angular motion of the gyroscope's case the torque required to keep the rotor at its null position is a measure of that applied angular motion.
Such a type II gyroscope system comprises a gyroscope, angle pick-offs arranged to sense the tilt angles of the gyroscope's rotor, control means arranged to receive a signal generated by the angle pick-offs and to generate a control signal and torque applying means arranged to apply a torque to the gyroscope's rotor in response to the control signal. The control means include a primary integrator arranged to integrate the signal received from the angle pick-offs to generate the control signal.
The defining characteristic of such a type II system is that it subjects the control signal to two stages of integration. The first integrator is implemented in the primary integrator of the control means and integrates the signal from the angle pick-offs to generate the control signal which is applied to the torque applying means. Typically the torque applying means are electromagnetic coils positioned towards the perimeter of the gyroscope's rotor. The other integrator is the gyroscope itself. The dynamics of the gyroscope are such that its output angle is proportional to the integral of the torque applied to the orthogonal axis.
A type II control loop is the highest order control system that can be designed to be unconditionally stable. If the control signal is subject to a further integrating stage giving a third order system then the loop cannot be designed to be unconditionally stable and the resulting system may, for example, be subject to unbounded oscillations upon start-up.